In numerous electronic communication applications, optical waveguides are used in various optical circuits. A typical optical waveguide may comprise a core layer and a cladding layer. Generally, the core layer of an optical waveguide has a higher refractive index, while the cladding layer has a lower refractive index. The optical energy is transported in the core layer by total internal reflection at the boundary of core and cladding layer. By adjusting the index difference between the core layer and the cladding layer, a high-confinement or low-confinement optical waveguide can be produced.
A high-confinement optical waveguide typically has an effective refractive index difference of more than 0.1 between the core layer and the cladding layer. In a low-confinement optical waveguide, the difference in effective refractive index is less than 0.1. A low-confinement waveguide may typically have dimensions in the range of 5 microns in its width, length, and/or height. In contrast, a high-confinement optical nano-waveguide may have typical dimensions smaller than 1 micron in its width and/or height.
Furthermore, a low-confinement optical waveguide is typically curved into a radius typically larger than 0.5 mm, because a radius smaller than approximately 0.5 mm may cause the optical energy to leak or radiate out. In contrast, a high-confinement optical nano-waveguide can be curved into a radius smaller than 0.5 mm and can be bent to a radius as small as 1 micron. Compact and efficient optical circuits, which are difficult to manufacture using low-confinement optical waveguides, can be made using highly-confined optical nano-waveguides. An optical circuit typically includes a plurality of optical waveguides which are connected or coupled together.
Several methods have been used for production of optical waveguides in the past. These conventional methods can be divided into three groups. In the first group of conventional methods for producing optical waveguides, an optical waveguides core is created in a thin film of optically transparent material by a lithographic process and an etching method. The lithographic process defines the geometry of the optical waveguide in a resist and the etching method transfer the lithographically defined optical waveguide into the thin film substrate. Various optically transparent thin films such as silicon dioxide, polymers, silicon, InP, and GaAs have been patterned to create an optical waveguide using this method. A cladding layer is deposited on the etched core layer using various deposition methods. Bayram Unal, et. al. in Appl. Phys. Lett. (86, 021110, (2005)) publication discuses a waveguide made of tantalum pentoxide, wherein the waveguide is produced by lithography and etching method to accommodate lasers. Furthermore, in another publication, D H Hensler et. al in Applied Optics (Vol 10, No 5, pp 1037, (1971)) publication discusses waveguides of tantalum pentoxide that are produced by an etching step.
In the second group of conventional methods for producing optical waveguides, a metal layer diffusion, such as titanium or proton exchange diffusion methods, has been used for defining an optical waveguide in ferroelectric crystals of lithium niobate, lithium tantalite, and silicon dioxide (i.e. glass). In titanium diffusion, a thin layer of titanium is deposited on the surface of the crystal, patterned using lithographic methods, and diffused into the crystal by heating the crystal to very high temperatures. RV Schmidt and IP Kaminov in Appl. Phys. Lett. (Vol. 25, pp 458, (1974)) publication demonstrated these waveguides on the surface of LiNbO3. The diffused metal layer raises the refractive index of the crystal and hence create a core layer. The cladding layer is the original crystal. In a proton exchange method the crystal is immersed in acidic solution and the lithium is replaced with hydrogen in a high temperature bath, which raises the refractive index of the crystal and creates the core of the waveguide. M. De Mitcheli, et. al. in Optics Letters (Vol 8, No 2, pp 114, (1983)) publication demonstrated waveguides based on this technique in lithium niobate. This method has been widely used for production of low-loss and low-confinement optical waveguides.
The third group of conventional methods for producing optical waveguides has been disclosed in the past for production of highly-confined waveguides in silicon. For example, K K Lee et al. in Optics Letters (Vol 26, No 23, pp 1888-1890, (2001)) publication demonstrated this method of fabrication for creating low-loss waveguides. In this method, an optical waveguide pattern is defined by optical lithographic methods and is transferred to an intermediate layer of silicon nitride. Instead of etching the silicon waveguide layer, an oxidation method in a high temperature furnace is used to selectively convert the silicon material into silicon dioxide. The silicon dioxide has low refractive index and can form the cladding region of the optical waveguide. The core layer of the waveguides can be formed by the original non-oxidized silicon layer using this method.
The metal diffusion based methods and the proton exchange methods provide low scattering loss waveguides in ferroelectric materials. However, these conventional methods create low-confinement optical waveguides, resulting in large optical waveguides which are not suitable for many novel optical circuits. Furthermore, the low-confinement waveguides are associated with large modulation voltage for electro-optical modulators, low efficiencies in nonlinear optical waveguides made from ferroelectric crystals, and low efficiencies in active optical waveguides that are made by doping crystals with dopants, such as erbium-doped optical waveguides.
By utilizing conventional etching methods, highly-confined optical waveguides can be produced. However, because the sidewall roughness scattering losses are very important in highly-confined optical waveguides, conventional etching methods present significant challenges. For example, an optical waveguide produced by using lithography and etching typically suffers from nano-meter scale sidewall roughness, which scatters the light out of the core layer of the highly-confined waveguides. Furthermore, the etching of ferroelectric materials such as lithium niobate and lithium tantalate is very difficult. For example, in many situations, it is not possible to achieve straight sidewalls required for highly-confined waveguides when the etching of certain ferroelectric materials are attempted.
For creating the cladding of an optical waveguide, conventional oxidation methods have been used, especially for silicon-based waveguides. However, silicon is not transparent in most of optical spectrum and has a very high refractive index, which complicates the fabrication of many optical circuits.
Furthermore, for coupling light from an optical fiber to a high-index contrast waveguide, several methods have been disclosed in the past. In one example, US2011/0013869A1 publication discloses a method for coupling lasers or optical fibers to an optical circuit using a plurality of small micro-lenses that are manipulated by a micro-electromechanical devices in order to achieve necessary alignment tolerances. This method for coupling the light to high-index contrast waveguides requires a plurality of small optical lenses and individual alignment for each lens, which is time consuming and expensive to manufacture.
In another example, US2010/0135615A1 and US7643719B1 publications disclose a coupling mechanism based on a graded-index (GRIN) lens, which is deposited on a substrate's surface and is etched into the substrate to form a GRIN lens in the vertical direction. In this example, a patterned edge is created and forms a curved surface for horizontal focusing in order to couple light from an optical fiber to a high-index contrast waveguide. The GRIN lens method disclosed in these publications requires a precise control of refractive index profile, and it is generally difficult to manufacture an exact refractive index profile with a high level of precision.
Other conventional methods for coupling light from an optical fiber to high-index contrast waveguides include using grating couplers. For example, U.S. Pat. No. 5,033,812 and U.S. Pat. No. 5,101,459 publications disclose a grating device formed on the surface of the device. In this example, the grating device is used to couple the light from an optical fiber or free space to the high-contrast waveguide. The grating coupler is polarization-sensitive and wavelength-sensitive, and it can be used to couple one polarization of light to the waveguide around a specific wavelength. Unfortunately, the fabrication of grating coupler is difficult because of very small feature size manufacturing requirements. In addition, there are light scattering-related losses associated with the grating coupler.
Another type of conventional coupling method is related to tapered waveguides for coupling of optical energy between an optical fiber and a high-index contrast waveguide. For example, U.S. Pat. No. 7,239,779B2 discloses a method to achieve optical coupling via transfer of energy between waveguides on different layers. This method related to tapered waveguides were used for coupling optical energy between an optical fiber and a high-index contrast waveguide.
It may be beneficial to device a novel method to produce a high-confinement and low-loss optical waveguide. In one or more embodiments of the invention, this novel high-confinement and low-loss optical waveguide may be used singularly or in combination with ferroelectric crystals such as lithium niobate and lithium tantalate. Furthermore, the novel method for producing the high-confinement and low-loss optical waveguide may enable or assist production of high-confinement and low-loss optical waveguides in ferroelectric crystals. Moreover, the novel method for producing the high-confinement and low-loss optical waveguide in accordance with one or more embodiments of the invention may resolve at least some problems that exist in manufacturing extremely small and precise gaps required for coupling of optical energy between high-confinement waveguides.
Furthermore, it may also be beneficial to devise a novel method to couple light to a high-confinement optical waveguide. Because the mode size of a nano waveguide is very small compared to the mode size of an optical fiber (i.e. typically less than 1 micron for a nano waveguide, compared to 10 microns for an optical fiber), the coupling efficiency from an optical fiber to a high-index contrast waveguide is very poor. One or more embodiments of the present invention discloses a novel method to couple light to a high-index contrast waveguide with at least some improved coupling efficiency.
In addition, it may also be beneficial to devise a novel method to produce an optical coupler that can be used to couple light from an optical fiber to a high-index contrast optical waveguide with high efficiency. In one or more embodiments of the invention, this coupler may provide significant advantages as a polarization and wavelength-independent device that can also be manufactured easily.